Field of the Invention
The present invention relates to electrophotographic photoreceptors. More specifically, the invention relates to an electrophotographic photoreceptor having high durability and providing high-quality images.
Description of Related Art
Recent demands on electrophotographic imaging apparatuses are a high printing rate, a reduction in size, and ease of maintenance. These demands have placed new demands, for example, a reduction in diameter (a reduction in size) and an enhancement of durability on photoreceptor drums of apparatuses for forming electrophotographic images. The photosensitive layer of an organic photoreceptor (hereinafter, also referred to as “photoreceptor”), which has been generally used as an electrophotographic photoreceptor is composed of a charge-transporting material, a binder resin, and other components and can be readily worn by mechanical load.
The photoreceptor is worn by friction with a cleaning blade and also loses its original electrical characteristics, such as chargeability and optical sensitivity, by the repeated charging and exposure in the image forming processes. These deteriorations cause defects in images, such as a reduction in image density, smudgy background noise, or image blurring in high-temperature and high-humidity environments. Local scratches occurring by wear of the surface of a photoreceptor cause defects in images, such as stripe noise due to insufficient cleaning, leading to a reduction in service life of the photoreceptor. Throughout the specification, the term “image blurring” refers to a phenomenon that electric discharge products, such as ozone and nitrogen oxide, hydrophilize the surface of a photoreceptor to cause disorder in an electrostatic latent image in high-temperature and high-humidity environments and thereby to form unclear toner images.
In order to enhance the durability of a photoreceptor, the photoreceptor should have improved wear resistance. Techniques of providing a surface protective layer onto the surface of the photosensitive layer have been investigated. For example, a proposed technique of providing high wear resistance to a surface protective layer is addition of a curable binder resin and inorganic microparticles to the surface protective layer.
Although the surface protective layer enhances the film strength of the photoreceptor, it causes reductions in electrical characteristics, such as an increase in residual potential, or image memory (a difference in image density, i.e., the history of density in images occurring depending on the photoreceptor cycle). In order to prevent a reduction in electrical characteristics and defects in images by a surface protective layer, a proposed technique involves addition of a charge-transporting material to the surface protective layer for providing charge transportability.
In surface protective layers that have been proposed, however, the compatibility between the charge-transporting material of a low molecular weight and the curable binder resin is low, which precludes charge transfer by the surface protective layer, increases the residual potential, and forms defects in images, such as a reduction in image density. In addition, the plasticizing effect of the low-molecular-weight charge-transporting material causes a reduction in wear resistance of the surface protective layer.
For solving these problems, proposed techniques involve addition of inorganic microparticles to a surface protective layer where the surfaces of the microparticles are modified with a compound having hole transportable groups (see, for example, Japanese Patent Laid-Open Nos. 2010-130471 and 2010-180079). In these techniques, inorganic microparticles surface-modified with an alkoxysilane having a hole transportable group are uniformly dispersed in a surface protective layer. The filler effect by the inorganic microparticles and the curable binder resin can enhance the wear resistance, and prevent image blurring in high-temperature and high-humidity environments due to electric discharge products, such as ozone and nitrogen oxide.
The inorganic microparticles surface-modified with a surface modifier (also referred to as “surface treating agent”) having a hole transportable group can facilitate the charge transfer (transfer of holes) in the surface protective layer. The addition of the particles therefore inhibits trapping of the charge in the surface protective layer and has advantages of inhibiting a reduction in sensitivity characteristics and reducing the occurrence of image memory.
In recent years, the use of electrophotographic imaging apparatuses in the quick printing field has been rapidly expanded, and higher durability and higher image quality are demanded in photoreceptors. These demands, however, cannot be sufficiently satisfied by the known techniques described above, and further reductions in image memory and image blurring are needed.
An object of the present invention, which has been made in view of the above-mentioned problems and circumstances, is to provide an electrophotographic photoreceptor that has high wear resistance, does not cause image blurring in high-temperature and high-humidity environments or image memory, and can form high-quality electrophotographic images.